The invention relates to an upflow reactor for the biological treatment of waste water. For purifying waste water, a multiplicity of methods are used, inter alia filtration and biological methods which make use of aerobic or anaerobic microorganisms. In particular, waste water that is polluted with organic impurities in dissolved and undissolved form is treated by means of the Upflow Anaerobic Sludge Blanket (UASB) method. The anaerobic sludge blanket of a UASB reactor contains various bacterial species, of which some convert and hydrolyze the undissolved organic impurities to form water-soluble substances—principally organic fatty acids. Subsequently thereto, the dissolved substances are converted to biogas by anaerobic microorganisms present in the sludge blanket, and in this manner the waste water is purified. Biogas is a gas mixture of the components methane and carbon dioxide and also hydrogen sulfide and other trace gases. If sufficient biomass is present for the degradation of the substances present in the water, the optimum hydraulic residence time for the water that is to be purified in the reactor is given by the degree of pollution, expressed, for example, as chemical oxygen demand (COD) and the desired degree of purity. It is known that under favorable conditions, even with hydraulic residence times in the range of a few hours, degrees of purity of greater than 90% are achievable.
However, a high degree of purity may only be achieved in the long term if it is possible to hold a sufficiently large amount of biomass in the reactor or to achieve growth of biomass. The growth rate of anaerobic or aerobic biomass is in the range of approximately 0.05·d−1 to 0.5·d−1. It must be ensured that at least as much biomass is newly formed in the reactor as is continuously flushed out. In the unfavorable case of high hydraulic throughput with a low COD concentration, even with flushing low, the biomass contained in the reactor can decrease, because the rate of formation for new biomass depends on the amount of substrate supplied and on the COD of the waste water. It is clear therefrom that the effective biomass retention is of decisive importance with respect to the performance of a biological reactor, in particular one such having an anaerobic biomass.
In known anaerobic reactors, owing to the intensive production of biogas, high upflow velocities of a plurality of m/h occur. If the microorganisms are not fixed to special supports by special culturing, the high upflow velocity causes a selection of species which form natural aggregates. This selection process is based on the fact that species that do not form aggregates are lighter, therefore are flushed from the reactor to a greater extent and are finally displaced by the aggregating species. This selection process extends over time periods of a plurality of months to some years and leads to the formation of a special sludge form which is generally termed granulated sludge or else “pellet sludge”. The “sludge pellets” have a settling velocity in water of 50 to 150 m/h, whereas sludge flakes settle at approximately 1 m/h.
Typically, the sludge pellets are spherical or lens-shaped granules having grain sizes of approximately 0.5 to 2 mm. The sludge pellets consist of a porous lime framework which is formed in the course of the selection process. The bacteria in this process colonize not only the surface of the sludge pellets—for example in the case of solid closed support granules of a fixed-bed reactor—but may be also found on the internal surfaces of the lime framework.
Owing to the activity of the microorganisms, biogas is formed which firstly rises as gas bubbles, secondly also adheres to the biomass. Owing to the partial enveloping of the sludge pellets with biogas, the specific gravity of the sludge pellets falls below the density of water and the sludge pellets rise. The upwardly buoyant sludge pellets are collected by appropriately arranged gas hoods and there gradually release the gas again. The detachment of gas from the individual sludge pellets is reinforced by the upwardly decreasing hydrostatic pressure in the reactor, in that the gas is compressible and expands with a decreasing external pressure. The gas bubbles adhering to the sludge pellet become larger with decreasing pressure. The friction and sheer forces that act owing to the upward motion in the water thereby have a greater contact surface and the detachment of the gas bubble from the sludge pellet is promoted. Owing to the detachment of the gas from the individual sludge pellets, the specific gravity of the sludge pellets increases again, and so they fall back into the lower region of the reactor, where the process begins again. Owing to the formation of gas and the detachment of the gas from the sludge pellets, a cycle of floatation and sedimentation is in operation.
For the conversion of organic impurities, mass transport or diffusion at the surface of the sludge pellets plays a decisive role. The strength of the diffusion stream of a certain substance is proportional to the decrease in concentration thereof from the waste water to the microorganisms in the sludge pellet. The sludge pellet is in part surrounded by an envelope of adhering biogas. The decrease in concentration and the diffusion are inversely proportional to the thickness of this adherent gas envelope. The conversion of organic compounds and, in association therewith, the efficiency of the purification method, may be increased by detaching as quickly as possible the biogas envelope adhering to the sludge pellets. It is sufficiently known that the gas envelope adhering to the sludge pellets may be reduced by high turbulence, i.e. by high velocity gradients. However, in this case, it is necessary to take into account that excessive circulations in the reactor and the associated mechanical sheer forces can have a long-lasting effect on the growth process of the sludge pellets or prevent it. In an extreme case, the fragile granules can even be destroyed. Accordingly, effective circulation or recirculation of the biomass with gentle gas separation is desirable.
DE 10 2005 050 997 A1 discloses a method and a reactor for purifying waste water that is polluted with organic impurities, by means of an upflow anaerobic sludge blanket (UASB). The biomass present as sludge or sludge pellet suspension is circulated, wherein the proportion of recirculated biomass to total biomass in the reactor per day is greater than 0.1·d−1, in particular greater than 2·d−1, and particularly preferably greater than 10·d−1. The reactor comprises a reactor tank, conduits, a waste water mixer, a first flotation separator and at least one further floatation separator for separating reactor water, biomass and biogas, one or more mixers for mixing biomass and biogas, and a gas separator for separating biomass and biogas.
EP 0 170 332 A1 (whose United States equivalent is U.S. Pat. No. 4,609,460) discloses a method and a device for the anaerobic treatment of waste water by means of UASB, in which a vessel is used, into the lower region of which the waste water that is to be purified is passed and from the upper region of which the purified waste water is conducted away. In the vessel, anaerobic microorganisms are active. Between the waste water inlet and the overflow for the purified waste water, there are situated in the vessel gas collectors in the form of hoods, that are stacked one above the other, the upper region of which is connected via a conduit to a gas-sludge separation unit. By means of the activity of the microorganisms, gas is generated that is taken up by the sludge, and so this floats upwards as what is termed scum. This scum is collected by the hood and gradually releases its gas again, so that it again becomes heavier and sinks back to the bottom as what is termed bottom sludge. The gas released by the sludge pellets rises further upwardly in the lines together with the free gas bubbles collected by the hoods and entrains in the course of this scum particles and liquid which are separated off in the gas-sludge separation chamber. The gas is expediently removed, while the entrained liquid which can also contain sludge particles passes into a falling conduit which leads back to the bottom of the vessel. The bottom sludge by this means should be vortexed on the bottom, which leads to a loosening of the sludge zone in the bottom region and improved mixing of the microorganisms with the incoming waste water. However, since water is relatively heavy, the amount of the waste water that is transportable by the ascending gas and thereby also the vortexing performance of the recirculated waste water are limited. In addition, it is known that waste water reactors of this type must have reactor heights of at least 11 m before the effect described occurs.
EP 0 244 029 A1 (whose United States equivalent is U.S. Pat. No. 4,758,339) describes a UASB reactor which is equipped with a device for separating the three phases water, sludge and biogas. The separation device comprises gas hoods which are connected via passage openings to a gas collection box, wherein the passage openings are arranged in the upper region of the gas hoods below the top of the hood. In addition, each gas hood is equipped in the interior with retaining boxes. The retaining boxes and the passage opening are designed such that a gas cushion is formed which acts as a barrier to water and sludge.
WO 99/51532 (whose United States equivalent is U.S. Pat. No. 6,478,963B1) teaches a method and a device for the anaerobic purification of waste water in a vessel receiving waste water and sludge, with gas formation. The gas that forms is collected by a gas collector and the circuit driven by the ascending gas is used for loosening the bottom sludge that has sunk to the bottom of the vessel. Owing to a gas-lift effect of the ascending gas, the bottom sludge is removed from the bottom by suction and conducted separately from the waste water into the upper region of the vessel and hack into the waste water.
EP 0 711 732 A2 describes a module for a reactor for the anaerobic purification of waste water which contains an upper overflow threshold for the purified waste water that establishes the water level in the module, a plurality of collection hoods for biogas that are arranged staggered over the entire module cross section having an outlet into a gas collection space and an upper take off conduit for the exhaust air which is not collected by the take-off hoods. Above the respective floatation separator, the biogas is conducted into a gas collection chamber. The biogas is taken off from the collection hoods via a short pipe.
Studies have found that only 10 to 20% of the biomass present in a reactor actively participates in the purification process. 80 to 90% of the biomass present delivers virtually no contribution to the purification of the waste water. Accordingly, the recirculation of biomass and also the gentle separation of biogas is of decisive importance for the efficiency of biological methods, such as, for example, the UASB method.
The known biological reactors have a fixed geometry. After a start-up period of a plurality of weeks to months, stable operating conditions are established in the reactor, wherein the operating parameters fluctuate within method-specific process windows. The expression “process window” in this case designates associated ranges of the polydimensional space of the operating parameters which include, for example, the content and recirculation rate of the biomass in the reactor, the inflow amount, the chemical oxygen demand (COD), the temperature and the pH of the waste water fed. The process windows are substantially determined by the type of biomass used, the waste water supplied and the reactor parameters. In particular, the biomass recirculation that is important for reactor efficiency virtually cannot be controlled.